In two words: online operations. In a paragraph: Forget
partitioning, row-based replication and events. The big reasons
most people are going to salivate over 5.1, and probably start
plans to upgrade now, are the online operations:

online ALTER TABLE for column rename, column
default value change, and adding values to the end of an
ENUM/SET

Online, table-based logging. No more need to
restart your server to enable or change the general or slow
query logs. You can have the standard file-based output or
choose a table format…which you can query.

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